The present invention relates generally to aerodynamic control surfaces for aircraft, and more particularly to an improved geared tab configuration for aerodynamic control surfaces. This invention is especially useful on STOL (short take-off and landing) aircraft.
STOL aircraft are characterized by low take-off and landing speeds combined with relatively high speed cruise capability. To provide the required low speed pitch maneuver capability requires a very powerful elevator. This elevator power is obtained by using a large chord elevator with larger than normal deflection angles. For example, certain STOL aircraft may require up to 35.degree. of trailing edge up (TEU) deflection during low speed operation. In cruise, however, normal elevator deflections seldom exceed 3.degree. to 4.degree. although critical flight maneuvers may require as much as 10.degree. to 15.degree. of elevator deflection. During cruise these design requirements result in an elevator that is overly sensitive yet requires an actuator that is more powerful than normal.
What is needed is a mechanism that would simultaneously (1) reduce the large elevator hinge moments typical of moderate deflections in high speed flight; (2) reduce the elevator sensitivity for small deflections also typical of cruise; but (3) not reduce the elevator power at maximum TEU deflection used only during low speed operation. Previous geared tab machanisms have always been designed to either (1) reduce hinge moments or (2) to increase control surface effectiveness, but never to provide a combination of all three of the benefits noted above.
The novel geared tab system presented herein provides tab deflection inversely proportional to elevator deflection in the zero to maximum trailing edge down (TED) position and zero to about 13.degree. TEU position. This reduces the elevator hinge moments and over-sensitivity for deflections typical of high speed flight. However, between 13.degree. and 35.degree. of TEU elevator deflection the tab motion reverses and the tab returns to the faired condition relative to the elevator at maximum TEU elevator deflection. This provides 100% elevator effectiveness at maximum TEU deflection as required for low speed operation. Tailoring elevator sensitivity and the hinge moment reduction can be controlled by appropriate variations in tab gearing and tab to elevator sizing. This design can result in an elevator actuator size reduction of 50 to 75 percent resulting in a substantial aircraft weight reduction.
It is an object of the present invention to provide an aircraft elevator geared tab system to reduce elevator sensitivity at high airspeeds while still providing maximum elevator effectiveness at low airspeeds.
It is yet another object of the present invention to reduce elevator actuator size and aircraft weight.
These and other objects of the present invention will become apparent as the detailed description of representative embodiments thereof proceeds.